If you want to allow customers to first fill a form before going to the checkout, then you will have to intercept the Bundler's logic of applying discounts.

The best way to make sure your customers have to accept the form is by turning off the bundler checkout logic through our JavaScript API and then call our JavaScript function to go to the checkout once the customer is ready to go to the checkout :) 
It will be best that you share this with your developer, so they can implement that. 
We have a documentation on how to do this here: https://bundler.app/javascript-api-documentation

To stop the Bundler checkout, simply use the window.bndlr.preventBundlerCheckout(); function.
And to apply discounts and go to the checkout, use the following function: window.bndlr.checkout();